The present invention relates to a screw vacuum pump provided with an oil stripper.
FIG. 3 shows a known oil screw compressor including a compressor body 21 rotatably accomodating a pair of male and female screw rotors meshing with each other, a discharge line 3 leading from a discharge port 2 of the compressor body 21, and an oil stripper 5 having a filter 4 therein and connected to the discharge line 3. The screw compressor further includes an oil return line 22 leading from the oil stripper 5 through an oil pump 13 to oil supply portions such as bearings, shaft sealing portions and rotor chamber in the compressor body 21. The oil thus returned from the oil stripper 5 to the compressor body 21 via the oil return line 22 is discharged again from the discharge port 2.
The oil supplied to the oil supply portions in the compressor body 21 for the purpose of lubrication, gas cooling, sealing, etc. is discharged together with a compressed gas to the discharge line 3. Then, the compressed gas is separated from the oil by the filter 4 in the oil stripper 5, and is fed from an upper portion of the oil stripper 5. On the other hand, the oil separated from the compressed gas is dripped to a bottom portion of the oil stripper 5, and is reserved in the oil stripper 5. Then, the oil is fed by an oil pump 13 to the oil supply portions in the compressor body 21, and is then introduced to the discharge port 2. Thus, the oil is circulated between the compressor body 21 and the oil stripper 5.
Assuming that an internal compression ratio .pi. i of the compressor body 21 is .pi. i=7, a discharge pressure is 7 kg/cm.sup.2 G in the case that a suction pressure is the atmospheric pressure. Accordingly, the discharge pressure is constant, and a gas flow rate at the filter 4 in the oil stripper 5 is therefore constant. Designing the size of the oil stripper 5 is dependent upon the constant gas flow rate.
Also known is an oil-free screw vacuum pump utilizing the above-mentioned compressor body 21 to be supplied with no oil into the rotor chamber in the compressor body 21. The oil-free screw vacuum pump is used in the fields (eg., semiconductor and food industries) wherein reverse flow of foreign matter to a vacuum suction side is not permitted. In this vacuum pump, a gap between the screw rotors and a gap between each screw rotor and a wall of the rotor chamber are not sealed by oil. Therefore, it is necessary to rotate the screw rotors at very high speeds, so as to reduce a quantity of gas leakage through the gaps.
In the other fields wherein reverse flow of foreign matter to the vacuum suction side is somewhat permitted, an oil screw vacuum pump similar to the compressor shown in FIG. 3 may be utilized. In the oil screw vacuum pump, the gap between the screw rotors and the gap between each screw rotor and the wall of the rotor chamber are sealed by the oil supplied into the rotor chamber. Therefore, the rotational speed of the screw rotor needs not to be made so high as in the above-mentioned oil-free type.
FIG. 4 shows such an oil screw vacuum pump having the same construction as the compressor shown in FIG. 3. Referring to FIG. 4, air is sucked from a vacuum tank 11 by a vacuum pump body 21a, and is compressed in the pump body 21a. Then, a compressed gas is discharged through the oil stripper 5 to the atmosphere.
Referring to FIG. 5, a suction pressure of the vacuum pump shown in FIG. 4 starts changing from 760 Torr (atmospheric pressure) to a predetermined partial vacuum which depends on the application of the vacuum tank 11.
Assuming that a volumetric flow of air to be sucked into a suction port of the screw vacuum pump is represented by V.sub.1 (m.sup.3 /hr); a volumetric flow of air to be discharged from the discharge port 2 is represented by V.sub.2 (m.sup.3 /hr); a suction pressure is represented by P.sub.1 (Torr); and a discharge pressure is represented by P.sub.2 (Torr), the volumetric flow V.sub.1 is defined according to the screw vacuum pump, and the following equation holds. EQU V.sub.2 =V.sub.1 .multidot.(P.sub.1 /P.sub.2)
At starting of the vacuum pump, the suction pressure P.sub.1 is 760 Torr which starts dropping, and the discharge pressure P.sub.2 is also 760 Torr which is constant. Therefore, the volumetric flow V.sub.2 at a point shown in FIG. 5 is expressed as follows: EQU V.sub.2 =V.sub.1 .multidot.(760/760)
When the suction pressures are reduced to 100 Torr at a point b and 10 Torr at a point c as shown in FIG. 5, the volumetric flows V.sub.2 at the discharge port 2 are as follows: EQU V.sub.2 =V.sub.1 .multidot.(100/760) EQU V.sub.2 =V.sub.1 .multidot.(10/760)
In this manner, the volumetric flow at the discharge port in the case of a vacuum pump is changed widely according to the suction pressure.
As previously mentioned, the size of the oil stripper 5 is dependent upon a gas flow rate on the discharge side. Accordingly, in the case that the suction pressure at the point c shown in FIG. 5 is used when designing the oil stripper 5, an air flow rate on the discharge side at the point a is 76 times that at the point c, and 7.6 times that at the point b. As a result, the oil is widely scatterd to the atmosphere side at points a and b. In contrast, if the suction pressure at the point a is used when designing the oil stripper 5, the size of the oil stripper 5 becomes very large.
In the case of the compressor, since a gas flow rate on the discharge side is substantially constant, proper selection of the oil stripper is easy. However, in a case of the vacuum pump, the gas flow rate on the discharge side changes up to 50 times or 100 times. Because of such a large change in gas flow rate, proper selection of the oil stripper size is difficult in both the case of small flow rate and large flow rate.